Phase II drug metabolizing enzymes sulfotransferases (SULTs) catalyzed sulfation is important in the regulation of different hormones and the detoxification of drugs and other xenobiotics. Sulfation also leads to bioactivation of procarcinogens leading to toxic effect. The long-term goal of this research project is to understand human SULT biological functions and to investigate their relevance to human health under physiological and pathological conditions. Specific aims in this proposal are as follows: 1. To Investigate Mechanisms of Catalysis, Substrate Inhibition, and Product Inhibition/Activation of Human SULTs. The proposed bypass ordered mechanism and related alternative mechanisms will be investigated using kinetic analysis, isotope exchange, sulfated active site amino acid residue identification, and site-directed mutagenesis. The information will have important implications for the prediction of biotransformation pathways. 2. To Investigate the Effect of Sulfated Drugs on Human SULT Catalytic Activities. The inhibition and activation effect of clinically important drug sulfates on catalytic activities of human SULTs will be investigated. E. coli expressed and purified human SULTs; human intestinal cytosols; and human Hep G2 and Caco-2 cells will be used for these investigations. The effect of clinical drugs on human SULT activities may interfere SULT normal biological functions in hormone regulation and xenobiotic detoxification. 3. To Define Oxidative Regulation Mechanisms of Human SULT1E1. Oxidative regulation of human SULT1E1 in Hep G2 and Caco-2 cells and redox thiol regulation mechanisms of purified human SULT1E1 will be investigated using enzyme assay, Western blot, RT-PCR, amino acid modification, site-directed mutagenesis, kinetic analysis, crystal structure analysis, and computer modeling methods. Knowledge on oxidative regulation of certain human SULT is important in understanding the ability of SULT functioning under physiological and pathological conditions. This proposal studies human SULTs. These studies will be significant in understanding SULT biological functions including hormone regulation, drug metabolism, xenobiotic detoxification and procarcinogen bioactivation. The knowledge will be important in understanding drug side effect, drug-drug interaction, drug development, and the potential roles SULTs play in cancer prevention and causation.